RCN 47 for pdf - Steve Briggs

RCN
A Budget Basic Bolt-on BMX Build
Ever since I started researching
recumbents on the internet, I have been
fascinated by how many people have
built their own, the innovative materials
used, and the ingenuity of the builders. The
bikes created match the building requirements
and skills of the builders. I have seen bikes
made of wood, bikes made without welding,
and bikes where just about every part of the
frame was machined by the builder.
My first recumbent was a mail order
BikeE, the only recumbent I could find at the
time which looked like it could fit my 5’ tall,
105 pound, 26” inseam, 36” x-seam self. I
was right. None of the other bikes I have tried
since have fit (Vision, Rans, Quetzal...) Riding
the BikeE around for a year taught me that I
preferred a more closed pedaling position than
BikeE offered, and that I wanted a smaller,
more compact bike as well. The Vision and
Rans SWB that I tried didn’t fit, but allowed
me to see that I would probably like a SWB
bike, but I wasn't sure. What if I made a mistake
and didn't like it? Budget considerations
also wouldn't allow me to do the serial/multiple
recumbent ownership thing—I just can't
afford it. I also couldn't envisage making my
own until I came across Kent Peterson's noweld
homebuilt SWB at www.halcyon.com/
peterson/bentkent.html. After seeing that a
bike could be knocked together using the
skills that I had (saw, file, drill), I decided to
give the project a go.
I started gathering supplies: a friend of
my brother's donated a Kuwahara BMX
frame, I found a junked bike in the trash to
cut up, I had an old backpack frame, some
wood, and assorted bike parts from past bikes.
I did have to buy some parts, like wheels and
forks and tires and chains. I think that I managed
to keep my costs to below $300 Can
(about $200 US).
After doing research on the internet and
picking the brains of several people on the
HPV list (thanks, guys), I had a design drawn
up for a high BB, ASS bike. What made this
BMX conversion so simple was the frame
itself, which has a gusset between the top and
down tubes, with holes cut out of it for lightness
(which is a joke, as the frame weighs
around 7 pounds. All I had to do was to bolt a
boom to this gusset and I had myself a SWB
frame.
I took a tip from Mark Colliton’s RCN
BMX conversions Flat Black article (reprinted
this issue) and flipped the frame upside down.
This laid back the head angle to a relaxed 70
16
by Ming Dinh
emari@intouch.bc.ca
degrees and lowered the seat height somewhat.
I used a 16” wheel in front, fitted with a 16” x
1-3/8” Primo. The fork offset was around
25mm, giving me a trail of 65mm. The handling
of this combination is very stable and predictable
and I'm very happy with how it rides.
The boom of the bike is made from a road
fork that has had the tips cut off (they make great
bottle openers) and the blades drilled to accept
two 4” x 3/16” bolts. I cut a couple of wooden
boards to fit exactly in the space between the
head and down tubes on either side of the gusset
and screwed them together through some
holes in the gusset. That held them in place.
Next, I drilled a hole for the forward-most bolt.
I squeezed the fork blades until they were parallel
and bolted them to the frame. I could then
swing the boom up and eyeball where I wanted
to position the BB for heel clearance. I decided
on a 24” BB height. I then marked off where to
position the second hole on my wood block/
gusset and finished drilling and bolting the
boom on. The nice thing about this setup is
that it allows me to play around with BB height.
For instance, I think I may want to experiment
with a higher BB height. Flipping the cut-off
fork around will give me a 25” BB height, due
to the orientation of the fork offset. If I want an
even higher BB height, I will just cut off some
more wood blocks and drill new holes for the
new boom position. The BB was made from
cutting up the junked bike frame. I believe that
this is where being short came in handy for me.
I have read other people’s accounts of how they
made their boom from a BB attached to a length
Recumbent Cyclist News #47
Ming Dinh and the BMX “Shopper”—Ming Dinh
of seat tube and had it bend. Because I'm
short, I was able to keep the boom length
short (around 12”) and boom flex (or bending)
hasn't been a problem for me. That may
also be due to knee problems forcing me to
remain very low powered in my pedaling.
The seat tube portion slips nicely over the
fork steerer tube (boom) to allow seat to
pedal adjustment. I used a seat clamp to hold
it on. It twisted when I put pressure on the
pedal, so I cut a shim from a plastic soda
bottle that solved the problem.
The seat was made from an old backpack
frame and plywood. The seat bottom
was made from a triangular piece of plywood
that I have trimmed repeatedly until there is
as little wood left as possible to still give me
a comfortable seat. This allowed me to get
a good shape to be able to easily put a foot
down flat at a stop. The mesh fabric that I
used for the seat back was window screening.
This is temporary as it really isn't strong
enough for the purpose, even when doubled.
I added a wooden seat horn to the plywood
to prevent sliding forward during pedaling.
Cushioning and ersatz suspension is provided
by a Thermarest Sportseat and a couple
of layers of 5mm closed-cell foam. I have
this cushion positioned in such a way that it
supports my lower back too, so when I am
riding over rough spots, I just lean forward
and let the bike rock and roll under me while
I sit totally suspended on a cushion of air. I
suspect this also works well due to my small
size.